And supporters think they drafted a ballot measure lawmakers can’t ignore — as they mostly did when Initiative 728 was approved in 2000 — without risking a run-in with the state Supreme Court.

They’ve worded this year’s measure, Initiative 1351, in a way that would make smaller class sizes a component of basic education which the state is legally responsible for funding.

That’s important because the Supreme Court in the McCleary case found the state is not living up to its obligation and ordered it to fully fund basic education by 2018.

The court is monitoring lawmakers’ progress. Backers think if the initiative passes and not enough is done to shrink the size of classes, lawmakers will have to answer to the justices.

By comparison, Initiative 728 essentially urged lawmakers to do the right thing but didn’t contain any means of forcing them to do so. Lawmakers later erased the initiative from the books.

“The State Supreme Court has given Washington leaders clear instructions to increase funding for education, so there is no excuse for Washington to stay 47th in the nation when it comes to class size,” Mary Howes, manager for the Class Size Counts campaign, said in a statement.

To qualify for the November ballot, supporters must turn in petitions containing the signatures of at least 246,372 registered voters by 5 p.m. July 3. The Secretary of State’s Office recommends initiative sponsors submit at least 325,000 signatures to allow for invalid signatures.

It aims to shrink the size of classes in kindergarten through 12th grade shrink by up to 20 percent over a four-year period.

Under the measure, the maximum number of students would be 17 in kindergarten through 3rd grade, and 25 in fourth through 12th grades.

Schools in which more than 50 percent of students are eligible for free or reduced-price meals, class sizes would end up at 15 students in grades K-3, 22 in 4th grade and 23 in grades 5-12.

The measure also requires the hiring of additional librarians, counselors, school nurses, teaching assistants and other staff.

Sen. Marko Liias, D-Everett, who authored the unsuccessful Senate bill, said the initiative will allow voters to show how important they think smaller classes are to a child’s education.

“Citizens will be saying to lawmakers that as you fund the McCleary decision, make this a top priority,” he said.

But the initiative carries a potentially large price tag and no means of generating revenue to pay for it.

It could cost nearly $2.9 billion in the next two-year budget to pay for the added teachers and staff, according to a fiscal analysis prepared for the House and Senate bills. Of that, $1.4 billion is the state’s share and the rest is what local school districts would fork out.

The tab would rise to $7.1 billion in the 2017-19 budget cycle of which $3.4 billion would be the state’s portion, the analysis concluded.

“This is a big deal,” said Sen. Bruce Dammeier, R-Puyallup, a former school board trustee and member of the Senate education committee.

“One of the challenges with initiatives is voters are not given a funding source. They don’t have to wrestle with the decision of whether to raise taxes or cut programs,” he said. “I hope that voters will understand the ramifications.”